On the relationship between cyclones and extreme windstorm events over Europe under climate change

The relationship between cyclones and extreme wind events over Europe under climate change conditions is analysed using global as well as regional climate model simulations. In this study, climate change simulations based on the Special Report on Emission Scenarios (SRES) A2 and B2 are used. Cyclone systems over the Northeast Atlantic and Europe are identified for the Hadley Centre global general circulation model HadCM3 using an objective algorithm based on the search of the maxima of the Laplacian of the mean sea-level pressure (MSLP). Cyclone tracks are recognized in a second step of the procedure. Extreme cyclone systems are defined via the exceedance of the 95th percentile of the Laplacian of MSLP for each system. Extreme wind events are defined similar by values above the 95th percentile of the daily maximum wind speed at the lowest model level and related to the core pressure of the nearest cyclone system. Although the overall number of modelled tracks is underestimated in the control period of the global model's simulation with present-day greenhouse gas forcing, compared to reanalysis data, realistic patterns of the track density over the investigation area are simulated. Changes occur in particular with respect to the A2 scenario for extreme cyclone systems, while for B2 the changes are less pronounced. Especially over western parts of Central Europe, the track density of extreme cyclones increases for A2, accompanied by a tendency towards more intense systems. With respect to the A2 scenario, a tendency towards more extreme wind events caused by deepening cyclones is identified for several regions of Western Europe such as Spain, France, United Kingdom or Germany. Additionally, the climate change signal in the regional climate model (RCM) HadRM3H is analysed. In accordance with the signal of the wind speed changes in the GCM simulation, the RCM reveals an increase of the 95th percentile of the daily maximum wind speed over extended parts of Western Europe related to the areas of increased track density of extreme cyclones under the A2 scenario. Changes with respect to the SRES B2 scenario are similar in their structure, but less pronounced in their amplitude.